Turbine airfoil of composite material and method of manufacturing thereof

ABSTRACT

A turbine blade having a root and a tip and an airfoil with a core section made of composite material and a protective sheath or layer joined to the composite core at a location or locations exposed to erosion is described with the composite core of the airfoil being continuous from a location in vicinity of the root to a location in vicinity of the tip of the blade and including a section which widens with distance from the rotational axis along the length of the airfoil and which ends at the location in vicinity of the tip of the blade. The protective sheath or layer is secured by interference fit with widening section of the core.

RELATED APPLICATION

The present application hereby claims priority under 35 U.S.C. Section119 to Swiss Patent application number 01043/11, filed Jun. 21, 2011,the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to turbine airfoils of composite material,particularly for steam turbines, and methods of manufacturing suchairfoils.

BACKGROUND

In the following description the term “turbine” is used to refer torotary engines having a rotating part and a stator part force coupled bya fluid medium such as water, steam or gas. Of particular interest forthe present invention are axial turbines comprising radially arrangedfixed stator blades or vanes alternating with radially arrangements ofmoving rotor blades. Movements are generally defined as movementsrelative to a casing or housing.

In large turbines, particularly steam turbines, the moving blades orairfoils are presently manufactured using steel or titanium basedalloys. In a multi-stage turbine, the size of the blades increases fromstage to stage. In the final stage of the largest low pressure turbinesthe height of a turbine blade can exceed one meter or more. While it isdesirable to increase the size of the turbine stages and therebyincrease its flow-off surface and efficiency, the properties of currentmaterials have reached theirs limits mainly because of the largecentrifugal forces acting on the rotating blades.

To overcome the barriers set by the materials properties of steel andtitanium, composite material airfoils have been proposed using mainlycarbon fiber based materials. Though a large number of such designs havebeen published, real-world applications of such composite blades arecurrently limited to gas turbines for advanced aircrafts engines.

One of the reasons which so far prevented large-scale adoption ofcomposite blades in the field of electrical power generation is the lackof resistance of the composite materials to erosion. Specifically in thefield of steam turbine blades, the material is subject to erosion bywater droplet condensing from the steam passing through the turbine overa long period of operation. Under the constant bombardment of thecondensate from the water steam, composite material erodes much fasterthan the currently applied metal alloys and is thus not suitable asairfoil material for large steam turbine blades.

As known solution to the general problem of erosion, the use ofprotective layers has been suggested since the early thirties in anumber of published patent documents such as the German patent DE 536278C2. For composite blades, protective layers are described for example inpublished United States patent application US 2008/0152506 A1 andpublished international patent applications WO 2011/039075 A1 and WO2010/066648.

Whilst the solution of applying a protective layer or coating may havethe potential of reducing erosion at the exposed parts of the turbineblade, further improvements are required to render composite airfoilsoperational. In particular, it is seen as an object of the presentinvention to improve the way the protective layer and the composite coreof a turbine airfoil are joined.

SUMMARY

The present disclosure is directed to a turbine blade having a root, atip, an airfoil with a core section made of composite material and aprotective sheath joined to the composite core at least in an area ofthe airfoil exposed to erosion. The composite core of the airfoil iscontinuous from a location in vicinity of the root to a location in avicinity of the tip of the blade and includes a section which widensfrom the rotational axis along the length of the airfoil and which endsat the location in the vicinity of the tip of the blade.

The present disclosure is also directed to a method of manufacturing ablade for a steam turbine. The method includes providing a corecontinuously extending from a location in the vicinity of a root sectionof the blade to a location in the vicinity of a tip section of the bladeand surrounding the core of composite material with a protective sheathof a different material. The method also includes increasing the widthof the core of composite material at least at the vicinity of the tipend of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIGS. 1A and 1B show a three-dimensional view and a horizontalcross-section of a conventional last-stage blade for a steam turbine.

FIGS. 2A and 2B illustrate schematically changes in the chord length ofa blade in accordance with an example of the invention; and

FIGS. 3A and 3B show a three-dimensional view and a horizontalcross-section of a last-stage blade for a steam turbine blade inaccordance with an example of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Introduction to theEmbodiments

According to an aspect of the present invention, there is provided anairfoil of a turbine blade, preferably the rotating blade, with a rootand a tip portion with the airfoil having a continuous core made ofcomposite material and being continuous from the root portion to the tipportion and a protective sheath joined to the composite core at alocation or locations exposed to erosion, wherein the core widens atleast in proximity of the tip portion.

According to a preferred embodiment of this aspect of the invention amovement of the protective sheath along the core in radially-outwarddirection is prevented by an interference fit between the widening coreand the protective sheath.

When referring to a radial direction, such direction is defined as theradial direction from the rotational axis of the turbine rotor.

In a preferred embodiment of this aspect of the invention, theinterference fit is achieved by a continuous or step-wide widening ofthe composite core with distance from the rotational axis. This wideningcan result in an increased length of the circumference of cross-sectionsof the airfoil taken perpendicular to the radial direction or, morespecifically to turbine blades, in an increase of the airfoil's chordlength or profile thickness in direction towards its tip. In a variantof this embodiment, the chord length, as a function of the radialdistance, has at least one section in proximity to the tip where thechord length increases as a function of the radial distance, and mayhave a first section where the chord length decreases as a function ofthe radial distance followed by the section where the chord lengthincreases as a function of the radial distance when looking along theblade from its root to the tip.

The airfoil is understood herein as the aerodynamically-formed part ofthe blade that causes the desired reaction force on the rotor when inmotion relative to the surrounding medium. The term excludes theso-called root part of the blade, which is used to anchor the blade inthe rotor, and any shroud part, which in some of the known blade designsare used to interlock with the shroud part of the neighboring blades toform a sealing or stabilizing circumferential ring around the assembledblades. The main orientation of the shroud is thus in circumferentialdirection, whereas the airfoil and any specific tip extensions of theairfoil, such as winglets, are oriented towards the flow direction ofthe medium. Other stabilizing parts, such as snubbers, are also notincluded in the term “airfoil” as used herein.

The shape of the new airfoil is typically a complex three-dimensionalshape, which widens towards the tip in what could be described as aninverted cone section. The inverted cone shape provides an interferencefit preventing any relative motion in the radial direction between theprotective sheath and the composite core, thus reinforcing anyadditional fixing between the two. But it is important to note that theinverted cone shape of the blade can have advantages beyond theimprovement to the stability of the joint between the protective sheathand the composite core.

In a preferred variant of this aspect of the invention the wideningsection of the core ends at the tip or very close to the tip of theairfoil. However, the location of its starting point along the length ofthe blade is very much subject to design consideration such asstability, weight and desired flow-off area.

In another preferred embodiment of this aspect of the invention theprotective sheath is joined with the composite core through a bonding oradhesive layer such as a glue foil. The new design of the blade inaccordance with the invention acts to support and, in case of itsfailure, replaces the foil to prevent a rupture or loosening of theprotective sheath.

In a further preferred embodiment the protective sheath is a metallayer. The layer is advantageously assembled from several parts, forexample a thick part designed to cover the leading edge of the blade andany adjacent areas of high erosion, a second part designed to cover thetrailing edge of the airfoil and two thin sheet-like parts to cover theremaining exposed areas on the pressure and suction side of the airfoil.

The manufacturing of a composite airfoil or blade in accordance with thepresent invention can generally follow the known manufacturing steps forcomposite blades. These methods include for example the use of strandsor woven pieces of the composite fiber material. In a preferredembodiment of this aspect of the invention, additional fiber material isadded at the tip of the airfoil to the strands or woven mats, which makeup the main part of fiber material for the continuous core. After addingfiber material at the tip, the core material is impregnated with thematrix material as in conventional designs.

These and further aspects of the invention will be apparent from thefollowing detailed description and drawings as listed below.

DETAILED DESCRIPTION

Aspects and details of examples of the present invention are describedin further details in the following description using the example of alast stage rotor blade for a steam turbine.

Shown in FIG. 1A is a schematic drawing of a last stage rotor blade 10for a steam turbine. The metal blade includes an airfoil 12 attached toa root section 11. The root section 11 is typically slotted intomatching grooves within the rotor (not shown). In a turbine, a pluralityof such metal blades 10, typically 50 to 60 for a last stage, arearranged around the rotor to form the rotor part of the stage. At itsdistal end or tip 121 the blade rotates in close proximity to the statoror stator casing (not shown). As in other parts of this specification,reference to a rotational movement is meant to designate the rotation ofthe rotor of the turbine within the stator unless specified otherwise.

A horizontal, i.e. perpendicular to the radial direction, cross-sectionalong the arbitrarily chosen line A-B of the airfoil 12 is shown in FIG.1B. The cross-section illustrates the leading edge 122 and the trailingedge 123 of the airfoil 12. Also shown are what is typically referred toas the pressure side 124 and the suction side 125 of the airfoil 12. Animaginary line 126 connecting the leading edge 122 with the trailingedge 123 is defined as the “chord” of the airfoil 12.

In generally, the chord can be regarded as one of the possible measuresdetermining the width of the airfoil. Other such possible measures arethe circumference of a horizontal cross-section of the blade or itsarea, the profile thickness, or the length of the middle or camber line127. It is possible to refer to the increase of any one of such measuresas an increase in width or widening of the airfoil. However, to increasethe length of chord or any equivalent thereof towards the tip of theblade is seen as the most effective way of implementing the presentinvention.

The widening by means of increasing the chord length is illustrated inFIGS. 2A and 2B. In FIG. 2A the chord length L is shown in relation tothe radial distance r to the rotor axis of the cross-section at whichthe chord length L is measured for a known shape of an airfoil and foran airfoil in accordance with an example of the present invention. Theformer known airfoil shape is represented by the dashed curve 21 and thenew blade design is represented by the solid curve 22.

Though the true shape of a modern airfoil is a complex three-dimensionalshape, the chord length of most modern last stage steam turbine airfoilsfollows the profile 21. This profile 21 indicates a continuouslydecreasing chord length as is characteristic of a general conical shape.For the new profile 22-1 however the chord length L increases—in theexample shown continuously—toward the tip of the blade. The increase orwidening can also start close to the root of the blade. Such analternative example of the invention is represented by the curve 22-2.

With the widening airfoil design in accordance with the presentinvention it is possible to reduce the number of blades in the laststage to below 40, and potentially below 35, even for the largest steamturbines.

Another way of visualizing examples of the present invention is chosenin FIG. 2B: The dashed shape 23 represents a two-dimensional projectionof a conventional airfoil onto the paper plane and the solid shape 24-1represents a two-dimensional projection of an airfoil in accordance withan example of the invention onto the paper plane. Again the root islocated at the bottom and the tip at the top of the shape. The solidshape 24-2 represents a two-dimensional projection of an airfoil wherethe widening begins already at a location close to the root.

Though the two-dimensional projection into the paper plane of FIG. 2Bflattens visually the three-dimensional shape of both types of blades,it clearly shows the widening of the new airfoils.

Any protective sheath which is wrapped around the core of the newairfoil near the tip is forced into interference fit by the widening ofthe core at the tip. This interference fit is particularly important incases where the protective sheath is joined with the core by a bondingor adhesive layer. In operation, such blades are exposed to hightemperatures and centrifugal forces and, if the bond fails, theprotective sheath could fly off the blade and damage other parts of theturbine.

The interference fit prevents the loosening of the protective sheathunless it disintegrates completely. The design counteracts the forcescreated by the fast rotation of the blade in operation.

Steps of manufacturing a composite blade in accordance with an exampleof the present invention are described in the following making referenceto FIG. 3A showing a perspective view of a composite blade and FIG. 3B,which shows a horizontal cross-section of a composite blade similar toFIG. 1A.

A composite airfoil or blade in accordance with an example of thepresent invention can for example be prepared using the vacuum infusionprocess known per se. In such a process a woven mesh of fibers is spreadin a die, which approximates the shape of the airfoil to bemanufactured. Additional fiber material is added at the tip of theairfoil to the strands or woven mats. After adding fiber material at thetip, the fiber material is impregnated under vacuum conditions in thedie with the matrix material such as resin.

Other known manufacturing methods such as hand laminating can be used.Variants such as prepreg or wet layup can also be applied. An overviewof known method to produce a core of composite material is published forexample in the international patent application WO 2011/039075. Howeverfor the purpose of the present invention and the sake of clarity, nofurther details of these standard methods are reiterated herein.

Once the composite core is prepared and metal sheets which make up theprotective layer are formed and cut or machined, both are joined in acombined process involving steps of welding and gluing as describedbelow when making reference to FIGS. 3A and 3B.

In FIG. 3A a perspective view is shown of a composite blade inaccordance with an example of the invention. The blade 30 includes theairfoil section 32 and a root part 31. The root part 31 ispyramid-shaped. The tip 321 of the airfoil section 32 of the blade 30 iswidened by the presence of additional fiber material 322 in the core.

A cross-section of the blade 30 along line A-B is shown in FIG. 3B. Itillustrates schematically the structure of the airfoil 32 including thecore 33 of resin-based material reinforced with carbon fibers. The core33 is completely wrapped in a protective sheath made of an erosionresistant material such as titanium or steel alloy. For ease ofmanufacturing the protective sheath is assembled from at separate parts.In the example shown, these parts include a leading edge cover 34, acovering sheet 35 for the suction side, a trailing edge cover 36 and acovering sheet 37 for the pressure side.

The airfoil 32 is assembled by joining first the edge cover 34, thecovering sheet 35 for the suction side and the trailing edge cover 36 atthe two welding seams 381, 382. The core material 33 surrounded by anadhesive layer 39 is then placed into the recess formed by the weldedparts and the covering sheet 37 for the pressure side is placed on topto completely enclose the core. The covering sheet 37 for the pressureside is joined at the welding seams 383, 384. These latter welding seams383, 384 are located at the edges of recesses machined into the leadingedge cover 34 and the trailing edge cover 36, respectively, to avoidthermal damage to the adhesive layer and the core during the weldingprocess.

The present invention has been described above purely by way of example,and modifications can be made within the scope of the invention. Forexample the sheath which surrounds the core can be replaced by a partialcover which while limited to sections of the core particularly exposedto erosion is shaped to maintain interference fit as described.

The invention also consists in any individual features described orimplicit herein or shown or implicit in the drawings or any combinationof any such features or any generalization of any such features orcombination, which extends to equivalents thereof. Thus, the breadth andscope of the present invention should not be limited by any of theabove-described exemplary embodiments. Each feature disclosed in thespecification, including the drawings, may be replaced by alternativefeatures serving the same, equivalent or similar purposes, unlessexpressly stated otherwise.

Unless explicitly stated herein, any discussion of the prior artthroughout the specification is not an admission that such prior art iswidely known or forms part of the common general knowledge in the field.

LIST OF REFERENCE SIGNS AND NUMERALS

-   -   last stage rotor blade 10, 30    -   root section 11, 31    -   airfoil 12, 32    -   tip 121, 321    -   leading edge 122    -   trailing edge 123    -   pressure side 124    -   suction side 125    -   chord 126    -   camber line 127    -   chord length L in relation to the radial distance r 21, 22-1,        22-2    -   two-dimensional projection of an airfoil 23, 24-1, 24-2    -   core 33    -   leading edge cover 34    -   covering sheet for the suction side 35,    -   trailing edge cover 36    -   covering sheet for the pressure side 37    -   welding seams 381, 382, 383, 384    -   adhesive layer 39

1. A turbine blade comprising a root, a tip, an airfoil extendingtherebetween with a core section made of composite material and aprotective sheath joined to the composite core at least in an area ofthe airfoil exposed to erosion, the composite core of the airfoil iscontinuous from a location in vicinity of the root to a location in avicinity of the tip of the blade and comprises a section which widensfrom the rotational axis along the length of the airfoil and which endsat the location in the vicinity of the tip of the blade.
 2. The turbineblade of claim 1 wherein movement of the protective sheath along thecore in a radially-outward direction is prevented by an interference fitbetween the protective sheath and the widening section of the compositecore.
 3. The turbine blade of claim 1 wherein the widening is continuousor step-wise.
 4. The turbine blade of claim 1 wherein the wideningincludes a lengthening of the airfoil's chord length towards the tip. 5.The turbine blade of claim 4 wherein the airfoil has a section where thechord length decreases as a function of the radial distance followed bya section where the chord length increases as a function of the radialdistance.
 6. The turbine blade of claim 4 wherein chord length increasescontinuously as a function of the radial distance from the location invicinity of the root to the location in vicinity of the tip of theblade.
 7. The turbine blade of claim 1 wherein the protective sheath isjoined with the composite core by a bonding or adhesive layer.
 8. Theturbine blade of claim 1 being a blade for a steam turbine.
 9. Theturbine blade of claim 9 being a last stage blade for a steam turbine.10. A method of manufacturing a blade for a steam turbine comprising:providing a core continuously extending from a location in a vicinity ofa root section of the blade to a location in a vicinity of a tip sectionof the blade; surrounding the core of composite material with aprotective sheath of a different material; and increasing the width ofthe core of composite material at least at the vicinity of the tip endof the blade.
 11. The method of claim 10, wherein the protective sheathis assembled from several segments bonded to the core by an adhesivelayer.